http: fix mocking of Netty in unit-tests

The recently updated Netty brings new internal behaviour.  Unfortunately,
the mocking necessary to unit-test pool behaviour means these internals
are exposed as sufficiently mocked Netty is needed.

This patch fixes the problem by adding a mocked HttpHeaders object
that Netty now requires.

Target: master
Requires-notes: no
Requires-book: no
Patch: http://rb.dcache.org/r/6718/
Acked-by: Tigran Mkrtchyan